First, the Z5 series doest support RAW.

Well that's just pathetic for a phone released this year.

Second, that's not a solution to the problem by any means.

To the specific issue you raised, it is. If you want to pick the noise reduction system, shoot in raw and then decide for yourself.

The camera in a phone is meant to take quick and easy pictures that you can see or share right away.

Maybe the front facing camera. The back camera is most people's primary camera these days, so a lot of photos that people take they actually care about and thus actually care about the quality. It isn't 2009, smartphone cameras are legitimate cameras and they actually matter. They aren't toys.

If I have to manually process every fucking picture in Lightroom/Snapseed before it even looks decent, that already defeats the point of a smartphone camera.

According to your own definition of what a smartphone camera is that you just plucked out of the air...

When I was indoor, even in good light situations (12:00 in the morning, plenty of light coming through the window), the phone would purposely force the aperture and ISO to ridiculous levels, so pictures came really dark... up to the point that the phone itself would realize this and trigger the flash!

Change the light meter mode to spot metering, then pick the location in the photo that you want it to meter from.

So even if I could collect the RAW data, many pictures were already wasted due to incorrect exposure at the time of shooting.

Automatic is great, but camera software isn't psychic. It cannot know what part of the photo you want metered all of the time, where you want focus to be, and what level of noise reduction. At some point you, the photographer, need to take some responsibility.